Frequency selective components are important for many electronic products requiring stable frequency signals or ability to discriminate between signals based on frequency diversity. These functions are difficult to reliably and repeatably realize in monolithic form together with other microelectronic components such as transistors, diodes and the like.
One approach to realizing frequency selective functions employs a mass allowed to vibrate in one or more dimensions (e.g., a pendulum). Such a mass is conveniently realized as a thin film supported at critical points, for example, peripherally or alternatively along one edge or end, forming a thin film resonator structure. Such structures provide clearly defined mechanical resonances having significant utility, for example, as filters and as frequency stabilizing feedback elements in oscillator circuits.
A significant drawback of previous thin film resonators has been the need to fabricate a free-standing thin film membrane. Typically, this is effected by forming a sacrificial layer followed by deposition of the thin film membrane. The sacrificial layer is then selectively removed, leaving a self-supporting thin film layer.
Alternatively, a substrate material having a thin film layer disposed thereon is etched from the back side to provide an opening extending up to the bottom of the membrane. This can be accomplished by use of etchants having etch rates sensitive to doping of semiconductor materials coupled with use of a surface layer of material having different doping than the bulk of the material, for example. Other options include employing a surface layer or layers of different composition and/or crystallographic form or orientation to provide a thin film layer following etching or other treatment to selectively remove some of the material immediately therebelow. A variety of such techniques are described in U.S. Pat. No. 4,556,812, G. R. Kline et al., "Acoustic Resonator With Al Electrodes On An AlN Layer And Using a GaAS Substrate" (Dec. 3, 1985); U.S. Pat. No. 3,313,959, J. G. Dill, "Thin-Film Resonance Device" (Apr. 11, 1967); U.S. Pat. No. 4,456,850, T. Inoue et al., "Piezoelectric Composite Thin Film Resonator" (Jun. 26, 1984); U.S. Pat. No. 4,502,932, G. R. Kline et al., "Acoustic Resonator And Method Of Making Same" (Mar. 5, 1985); U.S. Pat. No. 4,460,756, J. S. Wang et al., "Method Of Making A Piezoelectric Shear Wave Resonator" (Feb. 3, 1987); U.S. Pat. No. 4,642,508, H. Suzuki et al., "Piezoelectric Resonating Device" (Feb. 10, 1987); U.S. Pat. No. 4,719,383, J. S. Wang et al., "Piezoelectric Shear Wave Resonator And Method Of Making Same" (Jan. 12, 1988); U.S. Pat. No. 5,011,568, S. D. Brayman et al., "Use Of Sol-Gel Derived Tantalum Oxide As A Protective Coating For Etching Silicon" (Apr. 30, 1991); U.S. Pat. No. 5,075,641, R. J. Weber et al., "High Frequency Oscillator Comprising Thin Film Resonator And Active Device" (Dec. 24, 1991); and U.S. Pat. No. 5,162,691, E. A. Mariani et al., "Cantilevered Air-Gap Type Thin Film Piezoelectric Resonator" (Nov. 10, 1992), which patents are hereby incorporated by reference.
An alternative approach involves forming a cantilevered beam capacitively coupled to adjacent structures (e.g., a conductor placed beneath the beam). The beam is free to vibrate and has one or more resonance frequencies. Disadvantages of these approaches include need to form free-standing structures and also a tendency of the beam to "stick" to adjacent structures if or when the beam comes into contact therewith.
A need to remove any sacrificial layer and/or underlying substrate material limits fabrication ease and results in structures which are extremely fragile with respect to externally applied forces. These factors contribute to reduced fabrication yields and reduced robustness of the finished resonator component.
What are needed are apparatus and methods for forming apparatus wherein the apparatus provides a thin film resonator having solid mechanical support and providing frequency selection characteristics.